Polymeric corrosion inhibiter for metal surfaces and the production thereof

ABSTRACT

The invention relates to a polymeric corrosion inhibiter for a metal substrate of formula (R 1 ) a A(R 2 X) b , where a=0-3, b=1-6, A is a bridge structure unit, R 1  is identical to or different from H or an aliphatic, aromatic, olefinic, cyclic, heterocyclic, polycyclic or polyheterocyclic radical, R 2  is identical to or different from a polymeric basic structure, X is identical to or different from a functional group selected from the groups comprising: carboxylic acids; dicarboxylic acids; polycarboxylic acids; carboxylic acid anhydrides; carboxylic acid chlorides; carboxylic sulfonic acids; and also esters, amides and imides of the aforementioned groups, the molar mass of the polymer lies between 200 and 10,000, preferably 400 and 6,000. The invention further relates to the use of a polymeric corrosion inhibiter for the treatment of metal surfaces and to a method for producing a polymeric corrosion inhibiter.

The invention relates to a polymeric corrosion inhibitor for a metallic substrate, to the use of a polymeric corrosion inhibitor for treating metal surfaces, to a method for treating metal surfaces, and to a method for preparing a polymeric corrosion inhibitor.

Corrosion on metallic components is a considerable problem and can lead to deterioration in the functionality of the component. Particular attention is paid to corrosion prevention in aluminum and its alloys, on account of the widespread incidence of aluminum in the air travel industry and partly also in the auto industry. The covering of the metal surface with a corrosion-inhibiting protective layer is known for the purpose of inhibiting or preventing corrosion. A particularly effective form of corrosion prevention for many metals, especially aluminum, has long been achieved by means of coating with chromate. Chromates are generally toxic, and chromium(VI) is considered to be carcinogenic. Presently, therefore, the use of chromium(VI) as a corrosion inhibitor is prohibited. Substitute formulations (e.g., chromium(III), zinc phosphate, other inorganic anticorrosion pigments, etc.) usually do not display the same good corrosion-inhibiting properties of chromium(VI), and/or in some cases are specific to particular metals, thereby ruling out the possibility of broad-spectrum, multi-metal protection. In the prior art, moreover, certain corrosion inhibitors face environmental and/or health concerns. There is therefore a need for alternative formulations which offer effective protection from corrosion.

Organic corrosion inhibitors are chemicals which adsorb strongly and in some cases specifically to metal surfaces and in some cases are able even to attach after chemical reaction. They may lead to certain sealing and so develop a barrier effect, or else inhibit anodic or cathodic corrosion reactions. These corrosion inhibitors are usually in the form of weak acids and their derivatives, which form insoluble salts on the metal surface.

The majority of organic inhibitors are relatively small molecules, which on the metal surface usually form thick, unorganized layers, where the mechanism of protection is based on physisorption. A disadvantage of this is that these layers are readily detachable by abrasion or act exclusively by virtue of their insolubility in water. In the latter case, the corrosion inhibitors have to be applied with solvent.

Organic corrosion inhibitors should be distinguished from conventional protective organic paint or coating layers. The latter usually have an inadequate barrier effect with respect to corrosion-promoting substances. For this reason, very predominantly, conventional paint or coating layers are not applied directly to the metal surface, but are instead usually applied to a primer layer, which generally comprises corrosion-inhibiting substances. The primer layer may be a coating layer which comprises corrosion inhibitors as additives.

Also known is the use of conductive polymers as corrosion preventive agents. A disadvantage of conductive polymers is that coatings can usually be applied to the substrate only at great cost and complexity (by means of electropolymerization, for example).

The object of the present invention is to provide a corrosion inhibitor for a metallic substrate that exhibits a good corrosion-inhibiting effect and is easy to apply.

This object is achieved by the subject matter of the independent claims. Advantageous developments are specified in the dependent claims.

The invention has recognized that a corrosion inhibitor of the invention on metal surfaces forms a thin polymer layer with long-term stability that exhibits a good corrosion-inhibiting effect.

Surprisingly it has been found that the polymer layer may comprise a substantially monomolecular, ordered, chemisorbed layer whose thickness is determined substantially by the size of the macromolecules. This could not have been expected, since the organic corrosion inhibitors of the prior art usually form thick, unorganized, physisorbed layers and the low mobility of polymers means that their reactions are normally fairly slow.

Thin, chemisorbed layers offer a number of advantages. Firstly, chemisorbed layers have long-term stability—which means that they are more difficult to detach abrasively in comparison to physisorbed layers. Secondly, relatively little corrosion inhibitor is needed in order to develop a corrosion-inhibiting film on the metal surface. Another advantage, for numerous applications, moreover, is that thin layers have less of an influence on the surface geometry than thick layers. With thick coatings, the surface geometry is determined largely by the coating. Geometric features of the metal surface with dimensions smaller than or comparable with the layer thickness are largely obliterated, generally speaking, by the coating. In the case of a thin layer with a thickness much less than the dimensions of the features, this does not occur to the same extent. Surprisingly, the corrosion inhibitor of the invention displays a largely nonselective effect on metals, and is therefore outstandingly suitable for multi-metal protection.

First of all, certain terms used in the context of the invention will be elucidated.

The term “corrosion inhibitor” identifies a substance which is able to slow down, inhibit, or prevent the corrosion of metals.

A polymeric corrosion inhibitor has at least one polymeric parent structure which has at least two interlinked monomer units. These may be the same monomer units (homopolymer), such as ethoxy units, for example, or different monomer units (copolymer), examples being ethoxy and propoxy units.

The terms “polymer”, “monomer unit”, and “monomer” are defined in the context of the invention in the same way as in the REACH Regulation (EC) No. 1907/2006, published Dec. 30, 2006 in the Official Journal of the European Union No. 396, p 54.

A polymeric corrosion inhibitor of the invention may have up to six polymeric parent structures, which are linked with one another via a structural bridging unit. In the context of the invention the term “structural bridging unit” denotes a nonpolymeric, organic structural element by which the at least one polymeric parent structure is linked. The starting point for the structural bridging unit may be a starter molecule for the polymerization, such as an alcohol or polyol such as glycerol or dipentaerythritol, for example, and the structural bridging unit need not necessarily be identifiable as a former starter molecule. The structural bridging unit may also be a C atom which links the at least one polymeric parent structure.

A polymeric corrosion inhibitor of the invention has the following formula:

(R¹)_(a)A(R²X)_(b)

-   -   where     -   a=0-3,     -   b=1-6,     -   A is a structural bridging unit,     -   R¹ identically or differently is H or an aliphatic, aromatic,         olefinic, cyclic, heterocyclic, polycyclic, or polyheterocyclic         radical,     -   R² identically or differently is a polymeric parent structure,     -   X identically or differently is a functional group selected from         the group consisting of carboxylic acids; dicarboxylic acids;         polycarboxylic acids; carboxylic anhydrides; carbonyl chlorides;         carbosulfonic acids; and also esters, amides, and imides of the         aforementioned groups, and     -   the molar mass of the polymer is between 200 and 10 000,         preferably 400 and 6000.

The molar masses stated are average molar masses, as usual for polymers.

In the context of the definition of the term “polymer”, a polymeric corrosion inhibitor of the invention may include by-products such as, for example, residual amounts of catalyst, or unreacted monomers. The invention has recognized that these by-products do not adversely affect the activity of the polymeric corrosion inhibitor.

The polymeric parent structure R² may preferably have ethoxy (EO), propoxy (PO), butoxy (BO), (CH₂)_(n)—NH—(CH₂)_(m) (with n and m from 0 to 10) units or mixtures of these units.

In one preferred embodiment the polymeric parent structure R² is a homopolymer of ethoxy units, a copolymer based on a mixture of ethoxy and propoxy units, or a homopolymer of propoxy units.

The parent structure R² preferably comprises 3 to 200, more preferably 5 to 100 monomer units. The polymeric parent structure can be branched, but is advantageously a linear parent structure.

The polymeric corrosion inhibitor advantageously has 2 to 6, preferably 2 to 3, more preferably 3 polymeric parent structures, i.e., based on the above formula: b=2-6, preferably 2-3, more preferably 3.

It is preferred for X identically or differently to be a functional group selected from the group consisting of carboxylic acids; dicarboxylic acids; polycarboxylic acids; carboxylic anhydrides; and also amides and imides of the aforementioned groups.

In one particularly preferred embodiment the functional group X is

—NR′—R″ or —O—R″,

where

R′ is an aliphatic, aromatic, olefinic, cyclic, heterocyclic, polycyclic, or polyheterocyclic radical or a derivatized acyl radical, and

R″=H or R′.

Polymeric corrosion inhibitors of the invention may be prepared by a method with the following steps:

-   -   a) providing a polymer of the formula

(R¹)_(a)A(R²Y)_(b),

where

-   -   a=0-3,     -   b=1-6, preferably 2-6, more preferably 2-3, especially         preferably 3,     -   A is a structural bridging unit,     -   R¹ identically or differently is H or an aliphatic, aromatic,         olefinic, cyclic, heterocyclic, poly-cyclic, or polyheterocyclic         radical,     -   R² identically or differently is a polymeric parent structure,     -   Y identically or differently is a functional group selected from         the group consisting of —OR, —NHR, and —NR₂, where R identically         or differently is H or an aliphatic, aromatic, olefinic, cyclic,         heterocyclic, polycyclic, or polyheterocyclic radical,         preferably identically H,     -   the molar mass of the polymer is selected such that the molar         mass of the polymeric corrosion inhibitor is between 200 and 10         000, preferably 400 and 6000,     -   b) esterifying and/or amidating or imidating the functional         group Y with carboxylic acids, dicarboxylic acids,         polycarboxylic acids, carboxylic anhydrides, carbonyl chlorides,         or carbosulfonic acids; and also esters, amides, and imides of         the aforementioned groups.

The polymeric parent structure R² of the polymer in step a) may preferably have ethoxy (EO), propoxy (PO), butoxy (BO), (CH₂)_(n)—NH—(CH₂)_(m) (with n and m from 0 to 10) units or mixtures of these units. The polymeric parent structure R² is advantageously a homopolymer of ethoxy units, a copolymer based on a mixture of ethoxy and propoxy units, or a homopolymer of propoxy units.

The polymers stated in step a) are commercialized (examples being the polyetheramines sold by the companies Huntsman and BASF) and/or may be prepared by processes known to the skilled person. The reactions of step b) are known from the literature.

The polymeric corrosion inhibitor of the invention may have functional groups on the polymeric parent structure that have not undergone complete reaction or conversion. The invention has recognized that this does not adversely affect the activity of the polymeric corrosion inhibitor.

The invention further provides for the use of a polymeric corrosion inhibitor of the invention for treating metal surfaces, preferably surfaces of a metal selected from the group consisting of iron, steel, aluminum, copper, zinc, magnesium, nickel, titanium, and alloys thereof. With particular preference the metal is selected from the group consisting of aluminum and its alloys.

Additionally provided by the invention is a method for treating a metal surface, comprising contacting the metal surface with a polymeric corrosion inhibitor of the invention. The metal is preferably selected from the group consisting of iron, steel, aluminum, copper, zinc, magnesium, nickel, titanium, and alloys thereof, preferably from the group consisting of aluminum and its alloys.

The corrosion inhibitor of the invention can be applied from solvent but can preferably be applied from an aqueous dispersion, or emulsion. Advantageously this may be a low-dose dispersion or emulsion. In one exemplary embodiment a dispersion may contain around 0.01 to 4 wt %, preferably 0.1 to 0.4 wt %, of polymeric corrosion inhibitor. In another exemplary embodiment an emulsion may contain about 0.01 to 4 wt %, preferably 0.1 to 0.4 wt %, of polymeric corrosion inhibitor.

The dispersion or emulsion may comprise additives. These, advantageously, are further corrosion inhibitors, such as inorganic pigments known from the prior art, examples being zinc pigments, Ca pigments, and aluminum pigments, in the form of phosphates, silicates, and/or combinations thereof. The dispersion preferably comprises zinc phosphate as additive. Other possible additives are other organic inhibitors such as sulfonates, phosphates, and phosphonates.

It is likewise possible for the corrosion inhibitor of the invention to be applied to the substrate by means of physical application techniques, examples being anodic and cathodic techniques, electrophoretic deposition coating, and thermal setting and fixing methods.

In the context of the method, a polymer layer having the following features is preferably applied to the metal surface:

-   -   a. a thickness of ≦50 nm, preferably ≦30 nm, more preferably         about 10 nm; and/or     -   b. an occupancy rate of the metal surface by the polymer of ≦2.5         mg/m², preferably ≦1.5 mg/m², more preferably ≦1 mg/m².

The invention has recognized that with the polymeric corrosion inhibitor of the invention, on the one hand, considerably thinner layers than are usual in the prior art can be formed on the metal surface, and, on the other hand, an improved corrosion inhibition effect is achieved.

A corrosion inhibitor of the invention may also be present as an additive in a coating or paint formulation. Coating materials which comprise corrosion inhibitor of the invention can be used as primers, for example, when treating metal surfaces.

Further provided by the invention is a metal workpiece obtainable by the above-stated method for treating a metal surface. The term “metal workpiece” encompasses any material known in the prior art that has a metal surface. The metal is preferably selected from the group consisting of iron, steel, aluminum, copper, zinc, magnesium, nickel, titanium, and alloys thereof. With particular preference the metal is selected from the group consisting of aluminum and its alloys. The metal workpiece advantageously has a polymer layer having the following features:

-   -   a. a thickness of ≦50 nm, preferably ≦30 nm, more preferably         about 10 nm; and/or     -   b. an occupancy rate of the metal surface by the polymer of ≦2.5         mg/m², preferably ≦1.5 mg/m², more preferably ≦1 mg/m².

The invention is elucidated below by means of working examples. First of all various measurement and testing methods are elucidated, followed by inventive and comparative examples.

I. Layer Thickness

The thickness of the polymer layer on the metal surface is determined by means of X-ray photoelectron spectroscopy. This method also allows inferences about the atomic composition of the layer.

II. Corrosion Tests

All of the corrosion tests were carried out using standardized sample panels:

-   -   a) sample panels from Q-Labs Deutschland GmbH, of type QD, made         of steel with the following material specification: ISO 3574         type CR1, CRS SAE 1008/1010, 0.5 mm thick, smooth surface     -   b) sample panels from Rocholl GmbH, of type AR, made of         aluminum, with the following material specification: ISO 209-1,         alloy 2024 T3 bare, 1.5 mm thick, uncoated surface

1. Salt Spray Test

The salt spray test was carried out in a salt spray chamber conforming to ISO 9227 (manufacturer: VLM GmbH) in accordance with DIN 51 021 SS.

2. Salt Storage Test

Sample panels were immersed halfway into salt water, i.e., 3% NaCl solution, at room temperature for a defined time period.

III. INVENTIVE AND COMPARATIVE EXAMPLES

Without restricting the general nature of the invention, the invention is elucidated in more detail below by means of a number of examples.

Example 1 Polymeric Corrosion Inhibitor 1

A polymer having an average molecular weight of approximately 4300 and the formula

(R¹)_(a)A(R²Y)_(b),

where

-   -   A=—CH₂—CH₂—,     -   a=0,     -   b=2,     -   Y is an OH group, and     -   the polymeric parent structure R² is an EO/PO block copolymer,     -   is catalytically esterified with an equimolar amount of a         polycarboxylic acid (polymaleic acid). The polymeric ester         carboxylic acid PEC is neutralized with amine, preferably         triethanolamine (TEA).

Polymeric Corrosion Inhibitor 2

A polymer having an average molecular weight of approximately 6000 and the formula

(R¹)_(a)A(R²Y)_(b),

where

-   -   A=—CH₂—CH—CH₂—,     -   a=0,     -   b=3,     -   Y is an NH₂ group, and     -   the polymeric parent structure R² is an homopolymer of propoxy         units,     -   is amidated with an equifunctional amount of polymaleic         anhydride. This polyamidocarboxylic acid PAC is neutralized with         amine, preferably triethanolamine (TEA).

Example 2

FIG. 1 shows a scanning electron micrograph of a freshly sanded type AR 2024 T3 bare aluminum sample panel subsequently coated with corrosion inhibitor 2. For the coating operation, the sample panel was immersed for an hour in an emulsion containing 0.4 wt % of corrosion inhibitor 2, and was subsequently dried at room temperature.

The scale is 10 μm, while the inhibitor layer thickness determined by means of XPS measurement is smaller by one thousandth.

Example 3

FIG. 2 shows four type AR 2024 T3 bare aluminum sample panels. Sample panels B and D were coated with corrosion inhibitor 2 as indicated in example 2. The thickness of the polymer layer is about 10 nm in each case. Prior to coating, the sample panels were freshly sanded with a 400-grit abrasive. Sample panels A and C were not treated with corrosion inhibitor, and serve as comparative examples. These reference panels were likewise freshly sanded. Panel B (inventive) was stored in salt water for 750 hours. Reference panel A was stored in salt water for only 250 hours. Plate D (inventive) was subjected to a salt spray test for 250 hours. Plate C (reference) was subjected to a salt spray test for only 100 hours. For the panels coated with the inhibitor of the invention, B and D, oxidation processes on the metal surface were largely suppressed, whereas a layer of aluminum oxide was formed on the reference panel. XPS measurements confirmed that the polymer layer showed virtually no change during the corrosion tests and that virtually no aluminum oxide was formed.

Example 4

The following three compositions were produced (C1: comparative formulation, F1 and F2 with corrosion inhibitor of the invention).

Product C1 F1 F2 (manufacturer) (wt %) (wt %) (wt %) Component A Acrylic Macrynal VSM 2896, 38.40 38.40 38.40 60% (Solutia, now Cytec) Solvent Methoxypropyl acetate 3.05 3.05 3.05 Solvent Butyl acetate 9.20 9.20 9.20 Solvent Solvesso 100 (Exxon) 3.05 3.05 3.05 Zinc phosphate 10.10 5.05 — (anticorrosion pigment) Corrosion — 2.00 — inhibitor 1 Corrosion — — 2.00 inhibitor 2 Magnesium Micro-Talc A.T. Extra 5.05 10.1 13.15 silicate (Mondo Minerals) TiO₂ Kemira 650 (Kemira) 5.90 5.90 5.90 Barium sulfate Blanc Fixe micro 14.15 14.15 14.15 (Sachtleben) Solvent Methoxypropyl acetate 4.20 2.20 4.20 Wacker HDK H 15 0.50 0.50 0.50 (Wacker) Milled using a bead mill, particle size <15 micrometers Component B Curing agent Desmodur N 75, 75% 6.40 6.40 6.40 (Bayer) Mix. 100 100 100

A layer 90 μm thick of each of formulations C1, F1, and F2 was applied wet to three steel sample panels and dried at room temperature. The thickness of the dried layer was approximately 60 μm. Then the layer and the metal surface were scored in a T shape (so-called T-cut). The purpose of this is to create an artificial defect and hence a point of attack for the corrosion, of the kind that may come about in the normal operation of a component, as a result of scratches in the paint, for example. The sample panels thus treated were subjected to a salt spray test for 504 hours.

FIG. 3 compares the sample panel treated with the comparative formulation C1 with the sample panel treated with the formulation F1.

FIG. 4 compares the sample panel treated with the comparative formulation C1 with the sample panel treated with the formulation F2.

Whereas the sample panel treated with the comparative formulation (anticorrosion pigment zinc phosphate) shows severe corrosive undermining of the coating film originating from the defect, there is at most only minor undermining of the coating film for the sample panels treated with formulations F1 and F2. 

1. A polymeric corrosion inhibitor for a metallic substrate, said polymeric corrosion inhibitor having the formula: (R¹)_(a)A(R²X)_(b) wherein a=0-3; b=1-6; A is a structural bridging unit; R¹ identically or differently is H or an aliphatic, aromatic, olefinic, cyclic, heterocyclic, polycyclic, or polyheterocyclic radical; R² identically or differently is a polymeric parent structure; X identically or differently is a functional group selected from the group consisting of carboxylic acids; dicarboxylic acids; polycarboxylic acids; carboxylic anhydrides; carbonyl chlorides; carbosulfonic acids; and also esters, amides, and imides of the aforementioned groups; and the molar mass of the polymer is between 200 and 10
 000. 2.-15. (canceled)
 16. The polymeric corrosion inhibitor of claim 1, wherein the molar mass of the polymer is between 400 and
 6000. 17. The polymeric corrosion inhibitor of claim 1, wherein the polymeric parent structure R² has ethoxy (EO); propoxy (PO); butoxy (BO); (CH₂)_(n)—NH—(CH₂)_(m) (with n and m from 0 to 10) units; or mixtures of these units.
 18. The polymeric corrosion inhibitor of claim 1, wherein the polymeric parent structure R² is a homopolymer of ethoxy units; a copolymer based on a mixture of ethoxy and propoxy units; or a homopolymer of propoxy units.
 19. The polymeric corrosion inhibitor of claim 1, wherein the parent structure R² comprises 3 to 200 monomer units; and/or wherein the index b is subject to the following: b=2-6.
 20. The polymeric corrosion inhibitor of claim 19, wherein the parent structure R² comprises 5 to 100 monomer units.
 21. The polymeric corrosion inhibitor of claim 19, wherein b=2-3.
 22. The polymeric corrosion inhibitor of claim 1, wherein X identically or differently is a functional group selected from the group consisting of carboxylic acids, dicarboxylic acids, polycarboxylic acids, carboxylic anhydrides, and amides and imides of carboxylic acids, dicarboxylic acids, polycarboxylic acids, and carboxylic anhydrides.
 23. The polymeric corrosion inhibitor of claim 1, wherein X=—NR′—R″ or —O—R″, wherein R′=an aliphatic, aromatic, olefinic, cyclic, heterocyclic, polycyclic, or polyheterocyclic radical or a derivatized acyl radical; and R″=H or R′.
 24. A method for treating a metal surface, comprising contacting the metal surface with a polymeric corrosion inhibitor of claim
 1. 25. The method of claim 24, wherein the metal is selected from the group consisting of iron, steel, copper, zinc, magnesium, nickel, titanium, and alloys thereof.
 26. The method of claim 24, wherein the metal is selected from the group consisting of aluminum and its alloys.
 27. The method of claim 24, wherein the polymeric corrosion inhibitor is applied from a dispersion.
 28. The method of claim 24, wherein a polymer layer is applied to the metal surface, said polymer layer having: a) a thickness of ≦50 nm, and/or b) an occupancy rate of the metal surface by the polymer of ≦2.5 mg/m².
 29. The method of claim 28, wherein said polymer layer has a thickness of ≦30 nm.
 30. The method of claim 28, wherein said polymer layer has a thickness of about 10 nm.
 31. The method of claim 28, wherein said polymer layer has an occupancy rate of the metal surface by the polymer of ≦1.5 mg/m².
 32. The method of claim 28, wherein said polymer layer has an occupancy rate of the metal surface by the polymer of ≦1 mg/m².
 33. A metal workpiece obtainable by a method of claim
 24. 34. A method for preparing a polymeric corrosion inhibitor, said method comprising the steps of: a) providing a polymer having the formula (R¹)_(a)A(R²Y)_(b) wherein a=0-3; b=1-6; A is a structural bridging unit; R¹ identically or differently is H or an aliphatic, aromatic, olefinic, cyclic, heterocyclic, poly-cyclic, or polyheterocyclic radical; R² identically or differently is a polymeric parent structure; Y identically or differently is a functional group selected from the group consisting of —OR, —NHR, and —NR₂, where R identically or differently is H or an aliphatic, aromatic, olefinic, cyclic, heterocyclic, polycyclic, or polyheterocyclic radical; and the molar mass of the polymer is selected such that the molar mass of the polymeric corrosion inhibitor is between 200 and 10,000, and b) esterifying and/or amidating or imidating the functional group Y with carboxylic acids, dicarboxylic acids, polycarboxylic acids, carboxylic anhydrides, and amides and/or imides of carboxylic acids, dicarboxylic acids, polycarboxylic acids, and carboxylic anhydrides.
 35. The method of claim 34, wherein b=2-6.
 36. The method of claim 34, wherein b=2-3.
 37. The method of claim 34, wherein R is identically H.
 38. The method of claim 34, wherein the molar mass of the polymer is selected such that the molar mass of the polymeric corrosion inhibitor is between 400 and
 6000. 39. The method of claim 34, wherein the polymeric parent structure R² has ethoxy (EO); propoxy (PO); butoxy (BO); (CH₂)_(n)—NH—(CH₂)_(m) (with n and m from 0 to 10) units; or mixtures of these units.
 40. The method of claim 39, wherein the polymeric parent structure R² is a homopolymer of ethoxy units.
 41. The method of claim 39, wherein the polymeric parent structure R² is a copolymer based on a mixture of ethoxy and propoxy units.
 42. The method of claim 39, wherein the polymeric parent structure R² is a homopolymer of propoxy units. 